Adds open-vm-tools, libdnet and oem-vmware packages

This PR continues on the work started by @crawford on porting open-vm-tools. Additionally, it adds the oem-vmware ebuild and a coreos-init change to add a SystemD network unit in order to enable receiving DHCP offers through broadcast, for VMware images **only**.

Except for the systemd network unit added in coreos-init, these three packages and their dependencies are only installed on VMware OEM images.

This PR is part of a larger change-set intended to create a good starting point for VMware OEM images. The change-set is composed of the following PRs: